We've shifted from a set of managers for individual patch releases to a
combined, collaborative Patch Release Team.  Update docs to match.

Signed-off-by: Tim Pepper <[email protected]>

GitHub doesn't enable concise tracking of the evolving state of PRs
as they shuffle along towards meeting merge criteria on a branch.
During the past 9 months we've evolved a process for this in the
patch release team so we can collaborate on triaging inbound cherry
picks and driving consensus on whether to accept them on a branch.
This new documentation attempts to capture the current process.  It
reads a lot more complicated than it is, although better tooling
would nevertheless be very welcome.

Signed-off-by: Tim Pepper <[email protected]>